The invention relates to an image intensifier comprising an evacuated vessel with a first disk which forms an input window and on whose inner surface a photocathode is disposed, and with a second disk which forms an output window and on whose inner surface a fluorescent screen is disposed, the spacing between the first and second disks being small in relation to the diameter of the image intensifier, the input and output windows being plane-parallel, and a spacer ring being disposed between the first and second disks whose inside diameter increases from the area of the first disk in the direction of the second disk.
Image intensifiers of this type, known as proximity-focused intensifiers or intensifier diodes, are distinguished by simplicity of construction, freedom from distortion of the image on the fluorescent screen, and, with a sufficiently high accelerating voltage and minimal spacing between photocathode and fluorescent screen, by good resolution that is uniform over the entire useful surface area.
The requirement for a high accelerating voltage and a minimal spacing between photocathode and fluorescent screen signifies high field strengths within the image intensifier, for the control of which various measures have already come to be known. For example, it is known to provide between the disks carrying the photocathode and the fluorescent screen a spacer ring whose inside diameter increases from the photocathode toward the disk carrying the fluorescent screen. (German Patent No. 26 52 070; Feinwerktechnik und Messtechnik, 1982, No. 2, pp. 59-61.) As a result, electrons released by accidental events at the interior walls of the image intensifier will not have an avalanche effect on the wall of the inner ring. Also, provision is made for all points on the inner surface of the spacer ring to be at a specific potential. To this end, the spacer ring is either fabricated from an electronically conducting glass or provided with an electrically weakly conducting surface.